[en] In order to study the effect of thermodynamic parameters of steam generator in fast reactor on the leak response of a small leak detection system, the author sets up the model of sodium-water products transport and diffusion in the steam generator. For the first time, a hydrogen line density concept is proposed, and the controlling differential equation of the hydrogen line density and hydroxide ion are established. In the end, the tri-dimensional transport and diffusion equation is established. By comparison between the computed results and measurement of the hydrogen detector response on H^- concentration in the sodium exit of steam generator, it shows that the model can exactly predict the transport and diffusion process of sodium-water reaction products on small water/steam leak in steam generator of fast reactor